In recent years, a wireless communication system with a carrier aggregation (CA) function in which a plurality of frequency bands are simultaneously operated as one communication line to transmit and receive data in a distributed manner for higher-speed data communication has been proposed.
In the wireless communication system with the CA function, a wireless base station establishes communication with a plurality of mobile stations. Further, the wireless base station is connected with a core network and other wireless base stations through a wired transmission path called an S1 interface or an X2 interface.
When the CA function is implemented, cells of a plurality of frequency bands overlap in the same area. A plurality of frequency bands used in the wireless communication system having the CA function are called component carriers. The component carriers are classified into a primary component carrier (PCC) serving as a main carrier and a secondary component carrier (SCC). A cell allocated as the primary component carrier is referred to as a main cell or a primary cell (P cell). A cell allocated as the secondary component carrier is referred to as a sub cell or a secondary cell (S cell).
A Physical Downlink Shared Chanel (PUSCH), a Physical Uplink Control Chanel (PUCCH), and a Sounding Reference Signal (SRS) are transmitted from a mobile station to the P cell side of the wireless base station, and the PUSCH and the SRS are transmitted from the mobile station to the S cell side. Here, the reason why the PUCCH is not transmitted to the S cell side is as follows. In other words, it is consequential to reduce a peak to average power ratio (PAPR) depending on a terminal. To this end, it is desirable to transmit data through consecutive bands. However, when the CA is performed, if the PUCCH is transmitted through a separate component carrier, data is likely to be transmitted through non-consecutive bands. In order to prevent this, when the CA is performed, the wireless base station transmits the PUCCH only to the P cell side.
The wireless base station accommodates a plurality of mobile stations within a communication coverage. For this reason, in order to establish communication with the mobile station, the wireless base station performs a process of selecting a mobile station of a communication target from a plurality of mobile stations, and then establishes communication with the selected mobile station. The selecting of the mobile station is also referred to as a scheduling process, and examples of the method include a maximum carrier-to-interference power ratio (CIR) technique or a proportional fairness (PF) technique. The mobile station selected by the scheduling process selects available uplink radio resources among radio resources of the P cell and the S cell, and establishes communication with the wireless base station.
Here, when the mobile station is moving at a high speed, a frequency of an uplink signal received by the wireless base station varies due to the Doppler shift (the Doppler effect). For example, when the mobile station is moving in a bullet train (Shinkansen), a frequency variation modeled in 3GPP TS36.104 Annex B.4 High Speed train condition appears.
As a method of improving reception characteristics in circumstances in which the mobile station is moving at a high speed as described above, for example, the use of a PUCCH format 2 is disclosed in 3GPP TS36.104 8.2.3 Requirements for high speed train. Since the mobile station transmits the PUCCH format 2 in order to notify of a channel quality indicator (CQI), the wireless base station can improve the reception characteristics by measuring a frequency variation using the signal and using a measurement result for signal reception.
Uplink communication data is transmitted through the PUSCH, but the mobile station uses the PUSCH only when there is communication data. For this reason, the PUSCH is not regularly transmitted from the mobile station. When the PUSCH is received, the frequency variation can be measured using the pilot signal included in the PUSCH, but frequency variation measurement accuracy is not enough since the number of samples for the measurement is small. For this reason, although the frequency variation is measured using the PUSCH, it is difficult to expect an improvement in the reception characteristics. Thus, the PUCCH format 2 regularly transmitted from the mobile station is often used for measurement of the frequency variation.
Here, when the S cell side establishes communication using the CA function, the PUCCH format 2 is not transmitted from the mobile station to the wireless base station. For this reason, it is difficult to secure desired reception characteristics in a state in which the mobile station is moving at a high speed. When the reception characteristics have deteriorated, in the wireless base station, the number of reception NG determinations of the PUSCH is increased, and the number of transmission processes increases accordingly. As a result, the throughput of the entire wireless communication system is decreased.
When the throughput is decreased, for example, it is considered to detect an increase in the number of reception NG determinations of the PUSCH and perform scheduling control in which radio resources of the S cell are not allocated to the detected mobile station.
Further, the following techniques have been proposed as a wireless communication system of a related art. First, there is a related art in which it is determined whether or not a handover is performed according to a speed of a mobile station when a cell of a moving destination candidate is more excellent in a communication quality than a cell of a moving source. Further, there is a related art in which execution of the handover is decided according to a moving speed while changing a parameter according to a type of communication. Further, there is a related art in which a cell to be preferentially selected, from a large-diameter cell and a small-diameter cell arranged to overlap, is decided according to a moving speed. Further, there is a related art in which, when there are a frequency band and an extension frequency band of a cell, allocation of radio resources is changed at a timing at which the extension frequency band becomes effective. Further, there is a related art in which, when some channels being used for communication between the wireless base station and the mobile station are not used for communication, the resources are used for a connection with another mobile station.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2013-70159
Patent Literature 2: Japanese Laid-open Patent Publication No. 2009-182413
Patent Literature 3: Japanese Laid-open Patent Publication No. 2002-27522
Patent Literature 4: Japanese Laid-open Patent Publication No. 2012-209649
Patent Literature 5: Japanese National Publication of International Patent Application No. 2002-542687
However, when the radio resources of the S cell are not allocated to the mobile station, it is thereafter decided whether or not the allocation of the radio resources of the S cell to the mobile station shall be resumed. For this reason, although the allocation of the radio resources of the S cell is stopped, the wireless base station consistently measures the radio quality of the S cell side. Further, the mobile station consistently transmits the pilot signal such as an SRS of the S cell side. Thus, when the radio resources of the S cell are not allocated to the mobile station, in the related art, an unnecessary process is performed in both the wireless base station and the mobile station. As a result, the wireless base station consistently has a high processing load and thus is likely to consume electric power unnecessarily. On the other hand, when the radio quality measurement is simply stopped in order to reduce the processing load and the power consumption, the allocation of the radio resources is not resumed at an appropriate timing, and thus the radio quality deteriorates.
Further, in the above-mentioned related arts, the return after the allocation of the radio resources of the S cell to the mobile station is stopped is not considered. Thus, although any of the above-mentioned related arts are used, it is difficult to improve the radio quality while suppressing the processing load and the power consumption of the wireless base station.